A major new study led by Greek paleoanthropologist Katerina Harvati, director of the Senckenberg Centre for Human Evolution and Palaeoenvironment (SHEP) at the University of Tübingen, together with researcher Mark Hubbe of the University of Tennessee, is reshaping our understanding of how the human lineage evolved.
Published in Nature Communications, the research challenges one of paleoanthropology’s most enduring assumptions: that the expansion of the human brain and the simultaneous reduction of the face and jaw were driven primarily by continuous natural selection.
The genus Homo, which includes modern humans, first emerged around 2.5 million years ago. Over time, its members developed progressively larger brains and smaller faces while exhibiting increasingly sophisticated behaviors, including more advanced stone tool use, geographic expansion, and complex social organization.
For decades, scientists attributed these parallel trends to steady natural selection, arguing that larger brains enhanced cognitive abilities, while smaller jaws and faces reflected reduced chewing demands as tools increasingly processed food.
The new study paints a more nuanced picture. The research team analyzed three-dimensional measurements from 87 fossil skulls representing nearly every well-preserved Homo specimen from the past two million years, including Homo habilis, Homo erectus, Homo heidelbergensis, Neanderthals, and early Homo sapiens. The dataset is among the most comprehensive ever assembled for the genus.
“We compared this exceptional dataset with six different evolutionary models using statistical analyses to determine which best explains the observed changes in cranial and facial morphology,” Harvati said. The models included natural selection, neutral evolution, evolutionary stasis, and punctuated equilibrium.
While the findings confirm the long-recognized trends of brain enlargement and facial reduction, they indicate that these changes cannot be explained by sustained natural selection alone. Instead, random genetic variation, stabilizing processes, and biological constraints appear to have played a far greater role than previously believed.
The most dramatic increases in brain size, particularly in Homo heidelbergensis and later in Homo sapiens and Neanderthals, seem to have occurred during periods when evolutionary constraints eased.
The researchers suggest that developmental biology, metabolic factors, and, above all, cultural innovation created the conditions for these evolutionary leaps. As Hubbe notes, bursts of technological and cultural change likely enabled our ancestors to meet the energetic demands of larger brains while fully exploiting the cognitive advantages they conferred.
